(1) Field of the Invention
The present invention relates to a method for preparing unsaturated compounds from paraffinic compounds and more specifically to a method for preparing unsaturated compounds mainly comprising monoolefins by an oxidative dehydrogenation of paraffins in the presence of a specific catalyst or a method for preparing unsaturated compounds mainly comprising .alpha., .beta.-unsaturated nitriles and monoolefins by ammoxidation of paraffins in the presence of a specific catalyst
(2) Description of the Prior Art
There has been known an oxidative dehydrogenation reaction of hydrocarbons as a method for preparing unsaturated compounds having a carbon-carbon double bond and there have been known two approaches to such an oxidative dehydrogenation reaction of hydrocarbons. The first approach comprises oxidatively dehydrogenating monoolefins to give diolefins. For instance, U.S. Pat. No. 3,274,283 discloses a method for preparing butadienes by oxidative dehydrogenation of butenes in the presence of a catalyst obtained by impregnating tin oxide with phosphoric acid to thus form phosphoric acid-supporting tin oxide. Subsequently, there have been proposed some methods for preparing diolefins in which a catalyst comprising an oxide of tin and phosphorus.
The second approach comprises oxidatively dehydrogenating paraffins to give olefins. For instance, U.S. Pat. Nos. 3,502,739; 3,801,671 and 3,927,138 disclose methods in which Group VIII metals are used as catalysts for preparing diolefins. In addition, a method for preparing monoolefins is disclosed in E.P. No. 0,189,282 (1986). The European Patent comprises oxidatively dehydrogenating ethane, propane or isobutane in the presence of a catalyst comprising an oxide of tin and phosphorus to form ethylene, propylene or isobutene respectively, the oxidative dehydrogenation being performed at a reaction temperature ranging from 200 to 700.degree. C. and a reaction pressure ranging from 1 to 50 bar. However, only the embodiment of the preparation of ethylene from ethane is clearly disclosed in Examples of the specification of the European Patent and the reaction temperature of 550.degree. C. is simply disclosed. High selectivity is achieved only when the reaction is performed at a high pressure. It is commonly believed that a complete oxidation reaction is predominant if the reaction is performed at ordinary pressure as described in J. Chem. Soc., Chem. Commun., 1986, p. 1058.
J. Catal., 1978, 52, p. 116 discloses an oxidative dehydrogenation reaction of ethane with an Mo-V catalyst and reports that the ethylene selectivity is 100% at an ethane conversion of 10% and the ethylene selectivity is 83% at an ethane conversion of 25%. Moreover, it reports that if propane is used as a starting material, the products comprise only acetic acid, acetaldehyde, CO and CO.sub.2 and any propylene is not produced.
As has been described above, setting aside the preparation of ethylene, there has not yet been proposed any catalytic system having high selectivity in the production of an olefin such as propylene or isobutene through an oxidative dehydrogenation reaction of an olefin.
Recently, Japanese Patent Unexamined Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 63-295546 which relates to an ammoxidation reaction of propane discloses an example in which a V-Sb-W/Al system as a catalyst is used and the patent reports that the propylene selectivity of 52.8% was attained at a propane conversion of 18.3%. However, the selectivity is still unsatisfied.
In addition, many attempts have been directed to the production of acrylonitrile by ammoxidation of propane noticing the difference in price between propane and propylene. However, a high reaction temperature in the order of 500.degree. C. is required in these methods and the addition of a halogen or sulfur compound is also required for improving the activity and selectivity of catalysts used, which in turn requires the use of devices and materials of high quality. As has been explained above, there has not yet been proposed any industrially acceptable method for preparing acrylonitrile from propane as a starting material irrespective of the difference in price between propane and propylene. The same may be said of the ammoxidation of butanes.